1. Field of the Invention
The invention relates to an exhaust gas system of an internal combustion engine, which is provided with an exhaust gas recirculation (EGR) apparatus which introduces some exhaust gas as EGR gas into an intake passage, as well as to a control method of that exhaust gas system.
2. Description of the Related Art
Widely known examples of an exhaust gas control apparatus that purifies exhaust gas discharged from an internal combustion engine include a filter that purifies exhaust gas by trapping particulates in the exhaust gas, and a NOx storage-reduction catalyst that purifies exhaust gas by storing nitrogen oxide (NOx) in the exhaust gas and then reducing that stored NOx.
With a filter, when the amount of particulates that the filter traps increases, the filter becomes clogged, which adversely affects the ability of the filter to trap more particulates, i.e., it adversely affects the ability of the filter to purify the exhaust gas. In order to recover that ability, a filter recovery process is performed in which the trapped particulates are burned off by supplying fuel into the exhaust passage and increasing the filter bed temperature. Also, with a NOx catalyst, as the amount of stored NOx increases, the exhaust gas purifying ability decreases. Therefore, in order to purify the exhaust gas by reducing the stored NOx, a rich spike process is performed in which fuel is supplied into the exhaust passage to temporarily make the exhaust gas air-fuel ratio rich. As a result, the NOx catalyst recovers the ability to purify the exhaust gas. Furthermore, with a NOx catalyst, a sulfur poisoning recovery process in which fuel is supplied to the exhaust passage is also performed to recover the ability to purify the exhaust gas, which has deteriorated due to sulfur poisoning. These processes which supply fuel into the exhaust passage are necessary to restore the ability to purify the exhaust gas to the exhaust gas control apparatus. Also, in order to prevent a fuel adding valve which is provided as a fuel supply device that supplies fuel into the exhaust passage from clogging, fuel is injected into the exhaust passage at predetermined cycles specifically to prevent clogging.
Many internal combustion engines provided with this kind of exhaust gas control apparatus are equipped with an exhaust gas recirculation (EGR) apparatus that removes some of the exhaust gas from the exhaust passage and introduces it as EGR gas into the intake passage in order to lower the combustion temperature so as to reduce the amount of NOx discharged. Japanese Patent Application Publication No. 2005-069207 (JP-A-2005-069207), for example, describes one exhaust gas system provided with an exhaust gas recirculation apparatus that removes EGR gas from downstream of a filter and introduces it into the intake passage. This system estimates the oxygen concentration downstream of the filter based on a change in the amount of particulates trapped by the filter, and performs control to reduce the flowrate of the EGR gas as the oxygen concentration decreases. Other such technology related to the invention is also described in Japanese Patent Application Publication No. 2001-234772 (JP-A-2001-234772).
With this kind of exhaust gas system, when fuel is supplied into the exhaust passage, the concentration of carbon dioxide in the exhaust gas that passes through the exhaust gas control apparatus such as the filter increases by the oxidation of that fuel or the oxidation of the particulates in the filter. In other words, the oxygen concentration of that exhaust gas decreases. As a result, the carbon dioxide concentration of the EGR gas removed from downstream of the exhaust gas control apparatus increases so the oxygen concentration of the intake gas that is drawn into the cylinder in a state in which that EGR gas is mixed with the air that is introduced into the intake passage changes. This change in the oxygen concentration of the intake gas results in combustion in the internal combustion engine becoming unstable.